rpc/model.go: Add a Copy() method to ListenSpec
rpc/model.go: Add a Copy() method to ListenSpec

This will be used to fix a device manager bug where the port in a listenspec
is unintentionally getting clobbered.

Change-Id: I4c8cb64672eac7221b352d6ae996334ba09a39b0

vanadium/go.v23,vanadium/go.v23,vanadium/go.v23